1. Field of the Invention
This invention relates to a catalyst temperature estimation device for estimating the temperature of a catalyst of a catalytic converter for converting exhaust to less harmful substances.
2. Description of the Related Art
A catalytic converter for converting harmful substances in exhaust gases, such as HC (carbon hydrate), CO (carbon monoxide) and NOx (nitrogen oxide), to less harmful substances is provided in an exhaust system of an engine. The temperature of the catalyst of such catalytic converter varies to a great degree depending on the operating state of the engine. Thus, it is necessary to control the engine appropriately, on the basis of the estimated value of the catalyst temperature, so that the catalyst temperature will not exceed the allowable temperature limit for the catalytic converter in any operating state. There is known a catalyst temperature estimation technique in which exhaust temperature in steady operation is stored, and exhaust temperature in transient operation is estimated by subjecting the stored temperature in steady operation to filtering.
When a fuel cut is performed for vehicle deceleration, a large amount of oxidation reaction heat is produced by a reaction in which O2 (oxygen) in the exhaust gases reacts with CO and HC adsorbed on the catalyst. Meanwhile, in a fuel recovery period after the termination of the fuel cut, a large amount of conversion reaction heat is produced by a reaction in which CO and HC in the exhaust gases are converted by O2 that was adsorbed onto the catalyst during the fuel cut. In either case, a rapid rise in catalyst temperature is caused, which makes it particularly difficult to estimate the catalyst temperature. The above-mentioned catalyst temperature estimation technique only estimates the rate of change of temperature in such transient operation to be smaller than that in steady operation, through filtering. Thus, especially when the fuel cut and fuel recovery is repeated frequently, there occurs a problem that the values departing much from the actual catalyst temperature are estimated, as indicated in dashed line in the time chart of FIG. 7.
A catalyst temperature estimation technique different from the above has also been proposed (see Japanese Unexamined Patent Publication No. 2004-263606, for example). In the technique disclosed in this publication, in the operating states other than that with a fuel cut, the temperature of exhaust from the engine is considered as standard catalyst temperature; from the standard catalyst temperature, exhaust pipe wall temperature is estimated; From the standard catalyst temperature and the exhaust pipe wall temperature, provisional catalytic-converter inflow exhaust temperature is estimated; then, catalytic-converter inflow exhaust temperature taking account of a drop in exhaust temperature due to running wind is estimated from the provisional catalytic-converter inflow exhaust temperature; and on the basis of this catalytic-converter inflow exhaust temperature, catalyst temperature is estimated. Meanwhile, in the operation with a fuel cut, this estimation process is suspended, and the value of the catalyst temperature estimated in the last processing cycle is used as the present value without change.
However, the use of the last value as the present value without change in the fuel cut period, as in the catalyst temperature estimation technique disclosed in the above-mentioned publication, means that the estimation is performed on the assumption that the catalyst temperature does not vary in the fuel cut period and the fuel recovery period. This results in great departure of estimated values from the actual catalyst temperature which shows a rapid rise due to the oxidation reaction of HC and CO, like the case of the conventional technique shown in FIG. 7, for example. Consequently, it is impossible to suppress a rise in catalyst temperature in the fuel cut period and the fuel recovery period, by performing engine control on the basis of the catalyst temperature estimated by this estimation technique. Accordingly, this estimation technique has a problem that the catalyst temperature rises beyond the allowable temperature limit, which causes damage to the catalytic converter.